As a method for producing a peptide, a ligation method is useful. Among such ligation methods, a native chemical ligation method (NCL method) is a method capable of producing a peptide having a natural amide bond (peptide bond) at a ligation site. Such NCL method may be applied to two unprotected peptide chains. This method has been known as a method useful for forming a natural amide bond at a ligation site (for example, Patent Document 1). As shown in the figure below, the NCL method involves a chemical selective reaction between a first peptide having an α-carboxythioester portion at the C-terminus thereof and a second peptide having a cysteine residue at the N-terminus thereof. In this reaction, a thiol group (SH group, which may also be referred to as a sulfhydryl group) on the side chain of cysteine selectively reacts with a carbonyl carbon of a thioester group, and as a result of a thiol exchange reaction, a thioester-bound initial intermediate is generated. This intermediate intramolecularly rearranges on a voluntary basis to give a natural amide bond to a ligation site. At the same time, the intermediate regenerates thiol on the side chain of cysteine. Using this reaction, it became possible to efficiently synthesize various polypeptides.

The main disadvantage of a typical NCL method is that either one of two peptide fragments to be ligated must have a cysteine residue at the N-terminus thereof, and that a peptide obtained after ligation also must have a cysteine residue at the ligation site in this method. Accordingly, in a case in which a desired peptide to be synthesized does not contain a cysteine residue, this method cannot be applied.
In addition, in a typical NCL method, two or more peptide fragments to be ligated are prepared by a solid-phase synthesis method, for example. When a peptide contains an extremely small amount of cysteine (or contains no cysteine), like a peptide existing in a living body, it has been necessary to prepare an extremely long peptide fragment to be subjected to such NCL method. Thus, it cannot be said that this is an efficient method.
On the other hand, it has been known that various glycopeptides and glycoproteins are present in a living body. The sugar chains of such glycopeptides or glycoproteins are broadly classified into two types; namely, N-linked sugar chains and O-linked sugar chains. N-linked sugar chain is generally a sugar chain binding to the nitrogen of amide on an asparagine side chain via an N-glycoside bond. In general, such N-linked sugar chain often binds to Asn in a consensus sequence -Asn-X-Ser/Thr- (wherein X represents an amino acid other than proline) in a natural state. O-linked sugar chain is a sugar chain binding to a hydroxyl group on a serine or threonine side chain via an O-glycoside bond. Examples of such N-linked and O-linked sugar chains will be given below (Gal: galactose; GlcNAc: N-acetylglucosamine; Man: mannose; Fuc: fucose; GalNAc: N-acetylgalactosamine). A natural glycopeptide having such O-linked sugar chain has been known to contain large amounts of proline, threonine, and serine (Non-Patent Documents 1 and 2).
    Patent Document 1: International Publication WO96/34878    Non-Patent Document 1: TRENDS in biochemical sciences, Vol. 27, No. 3, March 2002    Non-Patent Document 2: Cancer Biology & Therapy 6: 4, 481-486, April 2007